Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Wed Feb 28 2018

Valid 12Z Sun Mar 04 2018 - 12Z Thu Mar 08 2018

Initially, an upper ridge will be centered across western Alaska
but will quickly push eastward given the cascade of upstream
shortwaves. The first of many impulses in the chain should
accelerate from the Bering Sea through mainland Alaska and into
the northeastern Pacific. Recent operational models and multi-day
ensemble spaghetti plot comparisons have shown a weaker trend with
this initial shortwave pushing toward western Alaska early Monday.
Eventually this system will stretch out and potentially further
weaken across the Gulf of Alaska by the following day. Where the
guidance begins to diverge is with additional longwave energy
sliding across the northern Bering Sea on Tuesday morning. The
12Z/06Z/00Z GFS runs remain slower ejecting this system downstream
relative to the stronger consensus solution. Differences in
ejection speed coupled with the degree of amplification further
complicates the forecast toward Day 6/March 6. While all
operational models depict some level of troughing pushing toward
Alaska during the Tuesday/Wednesday timeframe, the global ensemble
means are more conflicted. More specifically, except for the 00Z
ECMWF ensemble mean, the others in the suite show low-amplitude
ridging which seems unreasonable given the evolving pattern. While
the guidance has struggled with this system, further upstream
features emerge which adds more uncertainty to the forecast by the
middle of next week. Overall, favored a general multi-model
consensus on Sunday before basing the forecast more heavily on the
00Z ECMWF/ECMWF ensemble mean. Did leave about 20 percent of the
06Z GEFS mean in the mix to account for general model differences.
However, did prefer the 00Z ECMWF suite given the level of
amplification favored in its ensemble mean solution.

Regarding sensible weather, there will be periods of snowfall
moving through western Alaska toward the mainland as the series of
shortwaves traverse the region. Amounts are modest at best
although the 12Z/06Z GFS runs depict a slight uptick in 12-hour
numbers relative to other solutions. Additionally, many deep
surface lows crossing the Bering Sea will significantly enhance
local wind fields over much of western Alaska. If some of the
operational models verify for the middle of next week, surface
wind gusts could exceed 50 to 60 knots at times across the far
eastern Aleutians.


Rubin-Oster